With the increasing prominence of disposable items, such as diapers, paper towels and the like, and in view of the widely prevalent use of absorbent tissues of various kinds, it has become important to obtain pulps of high absorbency and especially pulps that display high absorbency upon multiple re-wetting.
Typically pulps that are used for hygienic absorption purposes such as baby diapers and the like are constructed with an outside “acquisition” layer, which is a layer of pulp of good bulking properties and good absorbency due to relative fiber stiffness. A bulky material will contain a high percentage of void spaces or pores. For an absorbent product, these pores are used to acquire, transport and store fluid. Longer, stiffer fibers make bulkier air laid webs with more pore volume. Fluids are more easily acquired and transported if pore volume or bulk is high. The “acquisition” layer is positioned between the baby's skin and the absorbent core of the diaper. An “acquisition” layer of proper characteristics and properties allows the liquid to pass quickly into the absorbent core upon repeated wettings and at the same time this layer transmits the liquid into the principal absorbent core that holds the liquid. In a similar manner, an incontinent or catamenial device may be constructed. Further, wound dressing material may be construed in a like manner. These devices are absorbency products which require pulps having intensive absorbency properties.
Still further, absorbent multiply papers such as household towels may be constructed of multiple layers or plies including a core layer and thus these plies may be tailored according to the use to which these goods are being subjected or for the purpose these are employed.
Products such as diapers when used with an outside “acquisition” layer and an interior principal absorbent core, are presently desirably constructed with the “acquisition” layer made from crosslinked pulps such as are illustrated by the following European Patent applications 0 427,316 A2 and 0 427,317 A2 all by Herron et al. and U.S. Pat. No. 5,137,537 by Herron et al. assigned to Proctor & Gamble Co. Further, Canadian Patent application 2,035,402, by Kokko based on U.S. priority application Ser. No. 07/473,404 and assigned to James River Corp. likewise discloses such pulps.
Cross-linked pulps are typically prepared using formaldehyde-based compounds. More recently, polycarboxylic acids, particularly citric acid, have been shown to be effective cross-linking agents. Cross-linked fibers display excellent wet stiffness. The cross-links physically restrict the uptake of water into the fiber wall. By doing so, the fiber retains, better than convention fiber, the characteristic stiffness of dry fibers. A web of cross-linked fibers, therefore, retains its bulk and pore volume when wet, which enhances fluid acquisition, especially with repeated wettings or insults. However, chemically cross-linked fibers are considerably more expensive than fibers which may be employed without any cross-linking. Moreover, pulps employed in prior art processes for cross-linking purposes are generally not available in sheeted form (rolls or bales of sheets).
Although pulps have been bleached under various alkaline conditions, bleaching schedules and bleaching treatment are by now those typically employed by prior art. Accordingly, a wide variety of such schedules are practiced—for the most part employing at least one or more alkaline steps at fairly high temperatures. In such sequences it has also been known to employ caustic solutions at lower temperature and then the same solution is used to bring up the temperature to or greater than a boiling point of the solution as shown in Canadian Patent 578,573 entitled “Purification of Wood Pulp” granted Jun. 30, 1959. In this patent the pulps so produced are used for dissolving pulps, i.e., making cellulose acetate and other chemical derivatives of cellulose. No description has been found concerning the improvements in absorbency, rewetting properties, stiffness of fibers, etc. as described herein for the pulps as used for the devices or products as illustrated herein. Moreover, the distinction between dissolving pulps and fluff pulp should also be noted.